1. Field of the Invention
The present invention relates to an air/fuel ratio control system for an internal combustion engine.
2. Description of Background Information
An air/fuel ratio control system is utilized to purify the exhaust gases from an internal combustion engine, and to provide improved fuel economy. Such a control system generally detects the concentration of oxygen in the exhaust gases by means of an oxygen concentration sensor, and executes feedback control of the air/fuel ratio of the mixture supplied to the engine based upon an output signal from the sensor, to maintain the air/fuel ratio at a target value.
An oxygen concentration sensor for such an air/fuel ratio control system is known (Japanese patent No. 58-153155) whereby an output is produced which is proportional to the oxygen concentration in a gas under measurement. This oxygen concentration sensor includes an oxygen concentration sensor element having a pair of oxygen ion-conductive solid electrolytic members, each in the form of a flat plate. These solid electrolytic members have electrodes formed on respective surfaces thereof, and are disposed mutually parallel with a gap portion between them, within the gas under measurement. One of the solid electrolytic members functions as an oxygen pump element, and the other functions as a sensor cell element for sensing the oxygen concentration ratio. A current is caused to flow between the electrodes of the oxygen pump element, within the gas under measurement, such that the electrode of that element facing the gap between the two members operates as a negative electrode. The oxygen in the gas within the gap is ionized by this negative electrode of the oxygen pump element, and the resultant oxygen ions migrate through the interior of the oxygen pump element to the positive electrode of that element, and are then released as gaseous oxygen. When this takes place, the concentration of oxygen in the gap becomes reduced, so that a difference arises between the oxygen concentration in the gap and that of the gas at the exterior of the sensor cell element, whereby a voltage is developed between the electrodes of the sensor cell element. By varying the level of current supplied to the oxygen pump element such as to maintain this voltage at a constant value, the current level will vary in substantially linear proportion to the oxygen concentration in the gas under measurement, assuming a constant operating temperature. This current level constitutes an oxygen concentration sensing value output.
However with such an oxygen concentration sensor whereby an output is produced in proportion to the oxygen concentration, manufacturing deviations will readily arise during manufacture of the oxygen concentration sensor element formed of the oxygen pump element and sensor cell element, per manufacturing lots. Measures taken to reduce these manufacturing deviations will result in unavoidable increases in production costs. If this is not done, then manufacturing deviations of the oxygen concentration sensors will result in the the output levels from the sensors inherently being respectively different, for an identical air/fuel ratio. Thus, it will not be possible to accurately judge the air/fuel ratio of the mixture supplied to an engine based upon the output level from such an oxygen concentration sensor.